Quantcast Figure 4-4.-Isothermal conditions

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thermal gradient condition, the layer depth is the depth of maximum temperature. Above layer depth, the temperature may be uniform, or a weak positive or negative gradient may be present. Layer effect is the partial protection from echo ranging and listening detection, which a submarine gains when it submerges below layer depth. Reports from surface vessels indicate that effective ranges on submarines are greatly reduced when the submarine dives below a thermocline, and that the echoes received are often weak and sound “mushy.” Figure 4-4.-Isothermal conditions. (constant temperature).    Refer to figure 4-4 as you read the following text. The surface layer of water in the figure is isothermal, but beneath this layer the temperature decreases with depth. This causes the sound beam to split and bend upward in the isothermal layer and downward below it. Remember, when no temperature difference exists, the sound beam refracts upward due to pressure. When the temperature changes with depth, the sound beam bends away from the warmer water. Under normal conditions the sea’s temperature structure is similar to that shown in figure 4-5. This structure consists of three layers as follows: 1. A surface layer of varying thickness with uniform temperature (isothermal) or a relatively slight temperature gradient. 2. The thermocline, which is a region of relatively rapid decrease in temperature. 3. The rest of the ocean, with slowly decreasing temperature down to the sea floor. If this arrangement changes, the path of the sound beam through the water will change. Layer depth is the depth from the surface to the top of a sharp negative gradient. Under positive Figure 4-5.-Normal sea temperature structure. 4-5

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